Tbcinterfaces - Motorola MC68824 User Manual

SECTION 7
TBCINTERFACES
The TBC must be interfaced to a host processor, buffer memory, and the physical layer of the
communication medium.
7.1
TBC-TO-HOST PROCESSOR INTERFACE
Figure 7-1 illustrates the connection of the TBC to a MC68020 host processor, while Figure 7-2
illustratres the interface of the TBC to iAPX80186. For detailed descriptions of the bus signals see
SECTION 5 SIGNALS.
7.2
NON-M68000 BUS INTERFACE
The TBC has a programmable mode which allows use with non-Motorola byte ordered memory
structures. Figure 7-3 illustrates, for example, the comparison between the Intel 8086 and the
II
Motorola M680'OO memory configurations.
Information stored in a data buffer is assumed to be organized in bytes. The first byte is byte 0
and the last byte is byte 4. The TBC can internally swap the data on the bus according to the
swap bit in the SET MODE 3 command during TBC initialization. The byte swapping option is
limited to data that is transmitted or received in the data buffers. Initialization table entries and
pointers and control data in frame descriptors MUST be organized to the Motorola standard.
Another consideration besides byte order is control line usage. In case of the Intel 8086 Family,
the address lines always provide a full byte address using AO. A signal called BHEN is used to
indicate a data transfer on D8-D15. As a simple interface to Intel type devices, the TBC signals
can be utilized as follows: UDS is used as BHEN, LDS is used as AO. In a similiar manner, it is
possible to map bus arbitration schemes. In this case, the Intel HOLD function is similiar to
Motorola bus request, and HOLDA can be treated as bus grant. BGACK also provides extra
information to the system. In cases where the TBC may be enabled for unlimited DMA burst
cycles, the user may want to create a capability for the processor to request the bus immediately.
This can be accomplished in a potentially disruptive manner through encoding BERR on the BEC
lines, or in a more predictable manner through HALT or RETRY encodings.
MC68824
MOTOROLA